As silicon device sizes becoming increasingly smaller, and as a minimum feature size of CMOS devices approaches and goes below the 0.1 micrometer regime, very thin gate insulators can be required to keep the capacitance of a dynamic random access (DRAM) capacitor cell in a 30 femptofarad (fF) range. For instance, if insulators are formed of silicon dioxide, it can be necessary to keep the insulators to a thickness of less than 2 nanometers (20 Å), and possibly even as thin as 1 nanometer (10 Å). Further, even if the insulating material is kept to a suitable thickness, it can be required to form a very high aspect ratio, or very tall polysilicon capacitor structure, to achieve a desired capacitance in the range of 30 fF.
A commonly-used dielectric material is silicon dioxide (SiO2). However, thin layers of silicon dioxide can have high leakage current density due to direct band-to-band tunneling current or Fowler-Nordheim tunneling current. Accordingly, high-k (dielectric constant) films such as TiO2, Ta2O5, and Al2O3 have received interest as being possible substitutions for silicon dioxide as dielectric materials in DRAM capacitors. The higher dielectric constants of high-k materials can allow the use of thicker insulators, which can have orders of magnitude less tunneling current than a thin insulator while still yielding the same capacitance value as the thin insulator.
A difficulty with the utilization of high dielectric constant insulating materials is that the materials can have poor interface characteristics with silicon, and a high density of interface states. Such interface states can cause poor reliability of a capacitor structure, in that they can charge with time under use conditions. The resulting electric fields can cause breakdown of the thin dielectric insulators.
Among the materials which may have application as substitute dielectric materials for DRAM capacitors are aluminum oxide, aluminum nitride, and aluminum oxynitride. Such materials can be referred to herein as AlO, AlN and AlON, respectively, with it being understood that the compounds are described in terms of chemical constituents rather than stoichiometry. Accordingly, even though aluminum oxide can be described herein as being AlO, the material would typically be in the form of Al2O3, and the designation AlO used herein indicates that the material comprises chemical constituents of aluminum and oxygen, rather than indicating a particular stoichiometry of such constituents.
Several pertinent physical characteristics of AlO, AlN and AlON are as follows. First, aluminum oxide is a direct band gap insulator with a band gap of 7.6 eV and a dielectric constant of from about 9 to about 12, depending upon whether the material is amorphous or crystalline. If the material is crystalline, the crystallographic orientation can also affect the dielectric constant. Aluminum nitride has a band gap of 6.2 eV, and amorphous aluminum nitride has a dielectric constant of from about 6 to about 9.6.
Work performed with AlN gate insulators indicates that AlN can be used as a gate insulator in MIS C-V structures on GaAs and silicon. Further, it has been shown that the deposited AlN films can be oxidized to form an aluminum oxide layer. Such oxidation can fill pin holes in the gate insulator to avoid shorted device structures, in a similar way that SiON insulators can be utilized in conventional DRAM capacitor cells.
Aluminum nitride films can be grown epitaxially on silicon utilizing metal organic chemical vapor deposition (MOCVD). Alternatively, aluminum nitride films can be deposited by RF magnetron sputtering. Regardless of how the aluminum nitride films are formed, they can is subsequently be oxidized by, for example, exposing the films to oxygen at a temperature of from about 800° C. to about 1,000° C. for a time of from about one hour to about four hours. The aluminum nitride films can be oxidized either partially or fully into Al2O3, depending on the initial thickness of the films, the oxidation temperature, and the time of exposure to the oxidation temperature.
The above-described methods for deposition of aluminum nitride would typically be considered to be high temperature methods, and would utilize temperatures of 1000° C. or greater. Processes have also been developed for deposition of aluminum nitride films which utilize temperatures of less than 1000° C. Such processes comprise nitrogen implantation into aluminum films, and can, for example, utilize ion beams of nitrogen having beam energies in the range of 200 eV to 6 keV, and current densities up to 50 μA/cm2. Such densities can be produced by a Penning source type ion gun with a magnetic lens. Also, aluminum nitride can be formed by MOCVD, or by electron cyclotron resonance (ECR) dual-ion-beam sputtering, as well as by ion-beam assisted deposition (IBAD) using a nitrogen ion beam energy of 0.1 keV, 0.2 keV, or 1.5 keV. Still other methods for deposition of aluminum nitride films include low-voltage ion plating with reactive DC-magnetron sputtering, and reactive sputtering.
Aluminum oxynitride can also be deposited by processes utilizing temperatures of less than 1000° C. For instance, aluminum oxynitride can be chemical vapor deposited utilizing AlCl3, CO2 and NH3 as reactive gases in a nitrogen carrier, with the films grown from the mixed gases at a temperature of, for example, from 770° C. to 900° C. Further, aluminum oxynitride films can be grown by electron cyclotron resonance plasma-assisted chemical vapor deposition.
Studies indicate that thin films of aluminum nitride, aluminum oxynitride, and aluminum oxide can be deposited by evaporation of aluminum nitride and simultaneous bombardment with one or both of nitrogen and oxygen. Also, aluminum nitride and aluminum oxynitride films have been prepared by ion assisted deposition, in which aluminum was electron-beam evaporated on a substrate with simultaneous nitrogen ion bombardment. Aluminum oxynitride films can also be formed by planar magnetron sputtering from an alumina target in a mixture of nitrogen and oxygen, and can be formed by reactive RF sputtering in a mixture of N2 and O2. Also, aluminum oxynitride diffusion barriers have been formed in a temperature range of from about 400° C. to about 725° C. by annealing silver/aluminum bi-layers on silicon dioxide substrates in an ammonia ambient.
Finally, aluminum nitride can be formed by plasma nitridation of metallic aluminum. The aluminum nitride can then be converted to aluminum oxide, or aluminum oxynitride, by exposure of the aluminum nitride to an oxygen plasma.
Aluminum nitride films have previously been grown on aluminum films by RF sputter etching the metallic aluminum films in an ammonia-rare gas plasma at temperatures near room temperature under relatively modest applied plasma voltages. The technique has been used to form oxide tunnel barriers on superconducting metals for Josephson devices. The process essentially uses the plasma to generate reactive ions which then interact with a metallic surface to form an oxide film. Electric fields and ionic charges can be present which can control and accelerate ion migration across a developing oxide film, with the thickness of the oxide film increasing as a logarithm of reaction time. A steady, slow rate of physical sputtering can be maintained by utilizing bombardment with inert gas ions such that the growing oxide plateaus in thickness. The particular thickness can depend on the oxide properties and the plasma conditions. The plateau value can be reached by using parametric values to grow a given thickness, and/or by growing the thickness to a value greater than a desired thickness and then subsequently restoring parameters which reduce the thickness to the desired thickness. The oxide films formed by such procedures can be exceptionally uniform in thickness and other properties.
It is possible to extend RF sputter etching techniques to formation of aluminum nitride at temperatures of less than 200° C. by utilizing an ammonia reacting gas rather than diatomic nitrogen (N2). The ammonia can yield charged ions, while diatomic nitrogen produces neutrals whose diffusion through nitride is unaided by the field across the nitride. The concentration of charged ions produced in plasmas containing ammonia gas can be much smaller than the concentration of charged oxygen ions produced in an oxygen-containing plasma. Accordingly, it can be desired to preclude oxygen from a nitridation plasma if it is desired to avoid forming dielectric films comprised predominantly of oxygen anions. On the other hand, if it is desired to form a film comprising aluminum oxynitride, it will be desired to introduce oxygen in addition to the nitrogen. One method of introducing oxygen in a low dose is to introduce the oxygen in the form of N2O.